Is certified translation of diploma an absolute necessary?

Only they can answer the question as it is their rules that apply. But I think you propose a good solution. Send them a "faithful" translation that you can prepare at little cost. Offer to send a certified copy if necessary, but mention that the cost of such a translation is high.

But you might also be able to have your university provide an English language translation or even to assure the recipient that your translation is accurate.

Each program has its own rules and requirements, so the only way to know is to ask them what is required exactly. It could be the case that a certified translation is only needed at a later stage, or not at all.

If your degree is from a French University and relatively recent, you should be able to request a Diploma Supplement which carries information in English about your degree. This could satisfy some requirements better than a non-certified translation (but again, you should ask).

You are entitled to receive your degree certificate in English directly from your university.

Graduates in all countries taking part in the Bologna Process have the right to receive the Diploma Supplement automatically, free of charge and in any major European language.

Source: https://ec.europa.eu/education/diploma-supplement_en